Ornamental articles are typically made of natural minerals (metals) and metal alloys. From the viewpoint of allergy prevention and weight reduction, Ti (which is a transition metal) and its alloys (e.g., Ti--Al--V) are employed for production of the ornamental articles.
However, ornamental articles made of metal materials are generally liable to corrode when wetted with sweat in direct contact with human skin or when exposed to salt water or rain and, therefore, have a poor corrosion resistance. Further, the ornamental articles are liable to be scratched, cracked or chipped by external shocks, as they are used in daily life.
Recently, there have been developed color ceramics such as zirconia and alumina which are superior in corrosion resistance and scratch resistance, and golden and silvery sintered alloys such as cermet alloys and superhard alloys which are obtained by sintering a carbide or a nitride (e.g., WC, TiC, TiN) along with an alloy of iron group metals (e.g., Ni, Co).
The aforesaid color ceramics and sintered alloys exhibit their unique single colors, providing poor aesthetic effects. For improvement of the aesthetic effects, the ornamental articles are subjected to a partial coating process or to a combination process for a satin finish and a mirror finish. However, the aesthetic effects are still unsatisfactory. In addition, these processes increase the production cost.
For surface protection, there has been proposed formation of a hard carbon film such as of diamond on the surface of a base composed of a metal material such as gold or an alloy thereof (Japanese Unexamined Patent Publications No. 62-180071 (1987) and No. 1-244705 (1989)). Further, coating of various non-metal bases with diamond (which is the most chemically stable substance) has been proposed for enhancement of abrasion resistance. The diamond film can be formed on a surface of any of the various bases by a thermal CVD method, a microwave CVD method, a plasma CVD method or the like.
Where a diamond film is formed on a metal base, however, the diamond film has a poor adhesion to the metal base, so that the diamond film is liable to be separated from the base due to thermal expansion or the like during a prolonged use. To overcome this problem, it has been proposed that an intermediate layer such as of silicon carbide is formed on the base and then a diamond film is formed on the intermediate layer. However, the adhesion of the film is still unsatisfactory.
The diamond film disclosed in the aforesaid publications is formed by the plasma CVD method or the like. The resulting diamond film has a high purity diamond structure with SP.sup.3 -hybridized carbon atoms. The high purity diamond film has crystal grains of great sizes (not less than several micrometers) because of remarkable diamond crystal growth. Therefore, diamond filming is difficult unless diamond is allowed to grow to a greater thickness.
In addition, the diamond film has a rough surface (Rmax&gt;1 .mu.m) and many voids because of its great crystal grain sizes, so that irregular reflection occurs on the surface. Therefore, an ornamental article formed with such a diamond film is less lustrous. Although the irregular reflection due to the surface roughness may be eliminated by polishing the film surface, a polishing process for mirror-finishing the surface of the diamond film requires quite a long time, resulting in a lower productivity.
As described above, the diamond film having great crystal grain diameters exhibits a reduced adhesion to the base, and includes great defects therein. Accordingly, there is a possibility that the diamond film is partially chipped due to collision or impact.
Where a diamond film is formed on a base composed of a ceramic or a sintered alloy, irregular reflection on the surface of the diamond film deteriorates the aesthetic effects as in the aforesaid case.
Since the diamond film has many voids therein, a metal component in the base leaches out of an ornamental article when the article is exposed to sweat, salt water or the like over an extended period. This may cause allergy symptom such as an inflammation in human skin in contact with the article.